The present invention relates to a method and apparatus for the direct heat treatment of medium- to high-carbon steel rods as they are leaving a hot-rolling mill.
Medium- to high-carbon steel rods made of hot-rolled billets are usually subjected to patenting before wire drawing so as to provide a fine pearlite microstructure having improved cold workability and increased tensile strength.
Lead patenting, the oldest known method of patenting, is a heat treatment wherein a rod heated to 850.degree. C. or higher is cooled by continuous drawing into a bath of molten lead having temperatures in the range of about 450.degree.-650.degree. C. The rod treated by this method has improved workability and mechanical performance. Air patenting wherein the rod is re-heated and subjected to controlled cooling by, for example, blowing cold air against it, is also used extensively as a convenient method.
However, these methods involve the re-heating of a rod that has been cooled to ordinary temperatures. High costs resulting from the additional equipment and manpower required for this purpose are inevitable.
A method of direct patenting has recently been developed. In this method, a hot rod coming from the rolling mill is directly subjected to controlled cooling so as to provide a product having properties comparable to those of the patented rod. This method of direct patenting is also used extensively since it eliminates the re-heating step and provides for operation at a lower cost.
While several techniques are used to implement direct controlled cooling in this method, the method of cooling the rolled rod within warm water having a constant temperature is preferred because it can be implemented with simple equipment and at low cost. Unlike cooling with cold water, this method employs "film boiling" wherein a uniform layer of water vapor forms on the surface of the hot rod, thus preventing direct contact between the rod and warm water. As a result, the cooling rate is slowed, and if the proper water temperature is selected, a cooling rate suitable for direct patenting is obtained, yielding a product having a fine pearlite structure.
The rod to be treated by direct patenting is made of a billet that has been produced by continuous casting while it is subjected to electromagnetic stirring so as to avoid the occurrence of microsegregation. However, it sometimes occurs that billets having microsegregation are formed as the starting material for rod production.
The part of a billet affected by microsegregation has such a great hardenability that if it is subjected to ordinary controlled cooling, a product with a martensite structure having no workability may form. Even a billet having no such microsegregation may produce a martensite structure if the cooling equipment and operation are such that the cooling rate varies greatly to cause locate excessive cooling.
The present invention further relates to an apparatus for direct heat treatment for producing steel rods having increased tensile strength and drawability by subjecting hot-rolled steel rods at a high temperature to controlled cooling with a coolant. This apparatus may be used in the production of medium- to high-carbon steel rods for use as springs and tensioning members, either twisted or untwisted, in prestressed concrete (PC).
The following methods and apparatuses for use in the above defined direct heat treatment of rods are widely known.
One of the best known methods is the Stelmor method wherein a spiral coil continuously expanded on a horizontal conveyor is cooled with an air blast. (See Japanese Patent Publication No. 154623/1967). This method provides a rod having a reasonably uniform quality without locally quenched portions. However, the cooling action of this method is rather weak and the resulting rod does not have a sufficient strength. A strong air blast may be used to achieve some increase in the rod strength, but even this strong air blast is unable to effectively cool the overlapping portions of adjacent turns of the coil, and this causes nonuniformity of the rod's strength.
Methods are also known wherein the rod is shaped into a spriral coil which is either wound in warm water (see Japanese Patent Publication No. 8536/1970) or continuously expanded into a sequence of partly overlapping rings on a horizontal conveyor moving through warm water (see Japanese Patent Publication No. 8089/1971). These methods provide rods having uniform quality if boiling water is used as the cooling medium. However, the product has an insufficient tensile strength of about 10 kg/mm.sup.2 lower than the value obtained by patenting through a lead bath.
A method has been proposed for providing the coolant with strong turbulence by blowing air into warm water (see Unexamined Published Japanese Patent Application No. 9826/1982), but even a rod treated by this method has a tensile strength which is 5-7 kg/mm.sup.2 lower than the value obtained by lead patenting.
It is known that stronger rods can be obtained by using subcooled boiling water (.ltoreq.95.degree. C.) as a coolant, and apparatuses that materialize this idea have been proposed. If stable film boiling were maintained, the desired heat treatment could be achieved, but in fact nuclear boiling is usually induced even at elevated temperatures higher than the pearlite transformation point, and the resulting local quenching causes a fatal problem, specifically, production of a martensite structure.
A method of direct heat treatment capable of providing the necessary and sufficient cooling rate involving only film boiling and without inducing nuclear boiling even in the case of using subcooled boiling water has been proposed in Japanese Patent Application No. 105558/1984. This method produces medium- to high-carbon steel rods that have a tensile strength comparable to that attained by lead patenting and which are highly uniform in quality and have improved drawability.
In accordance with the method proposed in that application hot-rolled medium- to high-carbon steel rod is treated by performing controlled cooling on spiral coils of the rod, which rod has an austenitic metallurgical structure and which is transported in the form of non-concentrically expanded rings in a generally horizontal direction. Specifically, the spiral coil is passed through a heat treating vessel containing a coolant of a gas bubble-water mixed fluid under strong turbulent action which contains a uniform dispersion of oxidizing gas bubbles and which is held at a temperature not higher than 95.degree. C. The coolant is caused to flow in a predetermined direction and at a predetermined rate so as to provide uniform cooling conditions for the coil along its entire length.
An apparatus has also been proposed for implementing such a method of direct heat treatment and is shown schematically in FIG. 1.
A hot-rolled steel rod 51 as gripped by pinch rollers 52 is pressed through a laying head 53 and deformed into a spiral coil. The coil is dropped on a horizontally positioned conveyor 55 in the form of a sequence of non-concentric rings 54. As they are carried on the conveyor 55, the rings, upon being are subjected to preliminary cooling, successively leave the terminal end of the conveyor 55 to be transferred to a horizontal conveyor 57 positioned within a heat treating vessel 56. The rings are then carried on an inclined conveyor 67 and accumulated in a collector 68 outside of the heat treating vessel.
As the rod 54 moves through the heat treating vessel 56, it is subjected to controlled cooling. For the purpose of controlled cooling, the heat treating vessel 56 is provided with a discharge pipe 62' at the delivery end. The pipe 62' is connected to a heat exchanger 65 and has a bypass that combines with an outlet pipe from the heat exchanger 65 so as to be connected to a warm water tank 64. For the purpose of controlled cooling, the warm water in the tank 64 is held at a constant temperature and is drawn by a circulation pump 66 to be fed into the heat treating vessel 56 through a supply pipe 62 connected to the vessel 56 at a point upstream of the point where the spiral coil 54 is dropped into the vessel 56.
An oxidizing gas supply pipe 60 is provided under the heat treating vessel 56, and an oxidizing gas is blown into the vessel through a plurality of vertical nozzles provided under the horizontal conveyor 57 in the longitudinal direction. The oxidizing gas blown in this way is reduced to tiny bubbles (diameter less than 1 mm) by means of bubble breakers 59 positioned close to the nozzles. Turbulence of the bubbles is effected by an agitator 69. The bubbles pass upward through the horizontal conveyor 57. Generally, the coolant flows in same direction as that of the direction of travel of the conveyor 57. The warm water held at a constant temperature is mixed with the oxidizing gas bubbles to form a gas-water mixed fluid which serves as a coolant for patenting the rod 54.
With the apparatus shown above, the necessary heat treatment is achieved by completing the transformation of an austenite structure to a pearlite structure while the rings of the rod 54 are carried on the horizontal conveyor 567 within the heat treating vessel 56 or while the rings are being transferred from such conveyor 57 to the inclined conveyor 67. Once the rings have been transferred to the horizontal conveyor 57, they will remain in contact with substantially the same positions with respect to the chains making up the conveyors 57 and 67 until the rings are discharged from the vessel 56 on the inclined conveyor 67. This means that the rod 54 undergoes heat treatment as it assumes the form of partly overlapping non-concentric rings without changing the positions of contact with the conveyor chains. As a result, the rod is cooled at varying rates and a product having uniform quality over its entire length cannot be obtained.
FIG. 2 is a CCT curve (continuous cooled transformation diagram) for a high-carbon steel and a part affected by segregation. In the FIG. 2, P.sub.s is a curve through points where austenite to pearlite transformation is started, and P.sub.f denotes a curve through points where such a transformation is finished.
In ordinary controlled cooling, the rod is cooled along the path PQR. Pearlitic transformation is started as the rod passes P.sub.2, but if it reaches the dashed line CD without crossing P.sub.f, the progress of the pearlitic transformation is arrested and the rod is supercooled to a point M.sub.s where the martensitic transformation takes over, resulting in the formation of a martensite plus pearlite structure. If the rod is quenched without crossing P.sub.s, a martensitic transformation occurs, thus producing a martensite structure.
With a homogeneous medium- to high-carbon steel rod, the desired patenting is accomplished by no more than 30 seconds of heat treatment, but if the rod includes microsegregation which contains C and Mn in amounts 1.2 to 2.0 times as much as in areas having no microsegregation, the rod has greater hardenability and takes a few minutes to complete the austentite to pearlite transformation.
Furthermore, as already mentioned, even a homogeneous rod may produce a martensite structure if great variations in cooling rate cause excessive local cooling, as is often encountered in controlled cooling with an air blast, cooling with an air-water mixture, or cooling with warm water, all of which methods involve the laying down of the rod in the form of a sequence of nonconcentric rings.